The goal of the proposed work is to understand at the molecular level the transport of molecules into and out of the eukaryotic cell nucleus. This work is divided into two areas of investigation: 1) A study of the transport and assembly of the small nuclear RNA (snRNA) U1 and its associated proteins. For this study we will determine which portions of the RNA molecule are necessary for assembly into RNP particles and which portions are needed for transport of the particles into the nucleus. We will ask, using modified U1 RNA molecules, whether transport through the nuclear pores can be sterically blocked. 2) In the second part of the proposed work, we will ask what features of the nucleus itself are required for transport of the model U1 RNP particle. This will be done using the new finding that nuclei can be reconstituted from any type of DNA when that DNA is injected into Xenopus eggs. Intermediates in the formation of nuclei can be used to identify when binding and transport of nuclear proteins occurs. We will also ask: a) whether there are developmental differences in nuclear transport in the early Xenopus embryo, b) whether nuclear transport is energy-driven, and c) whether transport of snRNP particles into nuclei can occur in vitro.